builder property Null safety
Called to build children for the sliver.
Should return null if asked to build a widget with a greater index than exists.
May result in an infinite loop or run out of memory if childCount is null
and the builder always provides a zero-size widget (such as
SizedBox.shrink()). If possible, provide children with non-zero size,
return null from builder, or set a childCount.
The delegate wraps the children returned by this builder in RepaintBoundary widgets.
final NullableIndexedWidgetBuilder builder;